1. Technical Field
The present invention relates to a control device of an engine having a function of estimating the state of combustion in a cylinder.
2. Related Art
It is said that misfire in diesel engines is generally likely to occur when the amount of oxygen is deficient due to too much exhaust recirculation gas or a deficiency of supercharging pressure or when intake air temperature or water temperature decreases.
Fuel injection timings of diesel engines are generally determined on the basis of required torque which is determined by two factors, that is, engine speed and the opening of an accelerator pedal. Therefore, in order to prevent misfire, in addition to the above described control, it is required to correct fuel injection timing in response to the internal state of a cylinder such as a deficiency of the amount of oxygen or a decrease in the temperature.
However, from the viewpoint of environmental protection such as exhaust gas regulation, the filling ratio of exhaust recirculation gas tends to increase every year. For this reason, the width of setting of fuel injection timing enabling appropriate driving is limited to a narrow crank angle range, and the degree of freedom of control narrows.
Also, misfire attributable to a slight change in the driving condition is more likely to occur. Combustion fluctuation according to misfire leads to deterioration of drivability according to a decrease in torque, or increase in the emission of un-burnt hydrocarbon. Therefore, it is preferable to avoid combustion fluctuation as much as possible. Combustion fluctuation means change or variation in the combustion state of each cylinder for each combustion cycle.
In Japanese Patent No. 2956456 (see paragraphs [0058] and [0059] on page 6, a paragraph [0085] on page 8, and so on), there is disclosed a technology for grasping combustion fluctuation during lean combustion driving, and estimating combustion fluctuation of each cylinder on the basis of a difference between the instantaneous value and average value of the angular acceleration of a crankshaft, and performing combustion control, thereby reducing the combustion fluctuation.
In order to grasp combustion fluctuation of each cylinder, it is effective to install a combustion pressure sensor capable of detecting the combustion state of a cylinder. However, installation of a combustion pressure sensor leads to complication of the structure of an engine and an increase in the equipment cost. For this reason, sometimes, it may be impossible to use this combustion pressure sensor.
According to an engine control method disclosed in Japanese Patent No. 2956456, on the basis of a difference between the instantaneous value and average value of the angular velocity of a crankshaft, the combustion state of each cylinder is estimated.
However, it is impossible to exactly grasp whether the degree of progress of combustion in a cylinder, that is, variation of combustion fluctuation of each cylinder only on the basis of the instantaneous value of the angular velocity of the crankshaft.
For this reason, an object of the present invention is to more exactly grasp the internal combustion state of a cylinder and perform control such that the combustion state becomes satisfactory while suppressing an increase in cost, without complicating the structure of an engine.